Ion beam sources are used in a variety of applications including ion assisted deposition, co-deposition of materials, ion sputtering and the like. The present invention can be and is advantageously useful in all of the above-mentioned applications. However, the present device has particular utility in connection with the fabrication of a thin film device through ion assisted deposition.
As is well known in the art, if ion beams are employed in an ion assisted deposition during the preparation of thin film devices, the resulting thin films are characterized by many desirable properties which are otherwise not present. The use of ion beam sources is known to improve stoichiometry and to improve adhesion. The technique of using ion beams as part of a thin film deposition activity has also been used to modify stress and density in the deposited film, consequently reduce water vapor absorption. In particular with regard to optical thin film coatings, the refracted index is increased and the optical thin film stacks possess greater (long term) stability.
As is explained in U.S. Pat. No. 4,710,283, it is believed that ion bombardment, of both the substrate and the material being deposited, removes atoms that are not firmly entrenched in the substrate while at the same time other atoms of the material are driven more completely into the substrate. In addition, greater surface mobility for condensing atoms is provided. It is believed that when an ion beam is employed during thin film deposition, a film characterized by denser packing and better adhesion results.
As is also explained in prior U.S. Pat. No. 4,710,283, in a number of ion involved applications, an ion source capable of producing ions over a broad area, in response to applied low energy and low current is required. It had been the practice to employ a hot cathode along with a magnetic multipole source in order to obtain large area beams of ions having sufficient current density and beam energy. Hot cathode sources are very versatile and can be designed to produce beams of different sized areas and different current and energy intensities. However, since the hot cathode devices employ hot filaments within the ionization discharge chamber to thermally supply electrons, these arrangements have relatively short lifetimes. The lifetime of a filament can be increased by increasing the thickness thereof. However, thicker filaments require higher heating currents, thereby requiring larger power sources. In addition, hot filaments have been found to require large ionization discharge chambers.
U.S. Pat. No. 4,710,283 describes a cold cathode system which provides substantially all of the desirable features of a hot filament, magnetic multipole source, while not requiring a large ionization discharge chamber and not being characterized by a short filament lifetime. As can be seen from this prior patent, the preferred embodiment of that device includes a permanent magnet that is formed in a circle or ring shape having a cross section in the shape of the letter "C" with the opening of the "C" facing inwardly. The top side of the magnetic ring is formed as a north pole and the bottom as a south pole (or vice versa) so that magnetic flux passes out of the top of the magnetic ring into the bottom. As a result, there is an enclosure formed by the heavy magnetic flux passing between the upper and lower ends of the C-shaped section of the magnetic ring. An annularly shaped anode is located within this enclosure and is shielded by the magnetic flux. Located below the magnet is a cathode element and at the top of the magnet is an aperture that may be open or covered by a screen. The cathode also includes a port which permits an ionizable gas to be inserted into the region within the magnetic ring so that when an electrostatic field is produced between the anode and cathode, the ionizable gas is transformed into a dense plasma and ions become available to pass through the aperture in the form of an ion beam.
Similar ion beam sources are illustrated in U.S. Pat. Nos. 4,652,795and 4,716,340. The disclosures of each of the foregoing patents are incorporated herein by reference.